Various methods of manufacturing a silicon single crystal have been proposed. Among them, a Czochralski method (hereinafter, referred to as a CZ method) is the most common silicon single crystal manufacturing method. In the CZ method of growing a silicon single crystal, polysilicon is melted in a crucible to form a silicon melt. Then, a seed crystal is deposited to the silicon melt, and the seed crystal is pulled up at a predetermined rotational speed and a predetermined pulling speed to grow a cylindrical silicon single crystal below the seed crystal.
A silicon wafer, which is a material forming a semiconductor device, is obtained by slicing and polishing the silicon single crystal. In order to maintain various characteristics of the silicon wafer at a predetermined level or more, the diameter of a cylindrical portion of the silicon single crystal is controlled so as to be within a predetermined range when the silicon single crystal is grown. This is very important to improve product quality and reduce manufacturing costs.
For example, as a method of controlling the diameter of the cylindrical portion when growing the silicon single crystal, the following has been proposed: a method of calculating a difference between the measured value of the diameter of a silicon single crystal that is actually pulled up and a predetermined diameter value, and feeding back the difference to the temperature of a silicon melt and the pulling speed to control the diameter, thereby approximating the diameter of the silicon single crystal to the predetermined diameter (see Japanese Patent No. 6234593 and Japanese Patent No. 5279174).
However, the above-mentioned method of measuring the diameter of the pulled silicon single crystal, calculating the difference between the measured value and the predetermined diameter value, and feeding back the difference makes it difficult to accurately control the diameter of the silicon single crystal. That is, even though a diameter measuring position is set in the vicinity of a solid-liquid interface, it takes a long time for the actual diameter of the silicon single crystal to approximate the set value in the method of changing the temperature of the silicon melt or the pulling speed after a variation in the diameter of the silicon single crystal that has already been pulled up is detected. Therefore, a wave occurs due to the variation in the diameter of the silicon single crystal, and it is difficult to maintain the diameter of the cylindrical portion of the silicon single crystal to be constant.
Further, a method of using a high-brightness zone (which is referred to as a fusion ring) generated in a solid-liquid interface when a silicon single crystal is pulled up to control the diameter of the silicon single crystal has been proposed (see JP-A-7-309694). The fusion ring is a ring-shaped high-brightness region that is generated so as to surround the silicon single crystal at the solid-liquid interface when radiation light from the wall of a crucible is reflected to the surface of a silicon melt that arises by the surface tension of the silicon single crystal during a pulling process. Therefore, the fusion ring is regarded as an inclined plane, and the angle of the inclined plane is continuously measured to detect a variation in the inclination angle of the fusion ring. In this way, a variation in the diameter of the silicon single crystal is detected.
Furthermore, a structure that controls radiation light from a crucible to detect the width or diameter of a fusion ring which is regarded as an inclined plane has been known (see JP-A-2005-41705).
However, in the method of measuring the angle or diameter of the fusion ring which is regarded as an inclined plane and detecting the diameter of the silicon single crystal on the basis of the measured results, the interface of the fusion ring, which is a detection target, is unclear, and it is difficult to accurately detect the width or diameter of the fusion ring. Therefore, it is difficult to feed back a variation in the width or diameter of the fusion ring to accurately control the diameter of the silicon single crystal.